1. Field of the Invention
The invention relates to a device for applying liquid adhesive, in particular hot-melt adhesive, comprising a reservoir for the adhesive, an application nozzle for the adhesive and a shutoff valve in the line from the reservoir to the nozzle which is constructed as slit nozzle whose slit emanates from a spreading chamber to which the adhesive is supplied through a passage which opens centrally into the chamber at an angle to the direction in which the slit adjoins the chamber.
2. Description of the Prior Art
Such a device for applying adhesive is known from U.S. Pat. No. 3,126,574. Admittedly, the known device does not have an exit slit as used for covering closed surfaces with adhesive but a plurality of adjacent exit bores extending parallel to each other in a common plane. These bores are however considered equivalent to an exit slit. Accordingly, the present invention is intended also to cover a configuration of the nozzle in which the slit is replaced by an equivalent array of bores with the aid of which instead of an adhesive layer a field of adhesive strands can be applied to the material to be coated, usually a web moved past beneath the nozzle. In the known device the spreading or dispersion chamber is formed by a bore. From said bore at an angle of about 45.degree. the thin outlet bores extend, said bores running parallel to each other in a common plane containing the axis of the bore and their diameter being a small fraction of the diameter of the spreading chamber. Into the centre of the spreading chamber a horizontally extending supply passage opens which is preceded by a valve means. Said valve means serves to interrupt the application of adhesive whenever the discharge means is raised from the material to be coated. In this manner an intermittent application of the adhesive is made possible.
The spreading chamber has a relatively large cross-section so that as is generally usual in the extrusion by means of wide slit nozzles it permits a good distribution of the adhesive across the width of the nozzle or the width of the row of exit bores. For the same reason on such nozzles the length of the exit slit is normally constant over the width and the spreading chamber extends parallel to the exit opening of the slit over the entire width thereof.
The known device is not very suitable for working at high relative speed between the material to be coated and the nozzle. This is due firstly because of the valve means used in the known construction. However, the valve means wll not be discussed in detail here because it is not the subject of the invention. Rapidly closing and opening valve means are known per se. A further defect of the known arrangement resides in that in particular when said relative speed is high and adhesive, in particular a hot-melt adhesive, is to be applied only intermittently over short lengths the application at the start of the section to be coated with adhesive and at the end thereof tends to be irregular.